precision mediump float;
uniform vec4 u_FragColor;
void main() {
  float d = distance(gl_PointCoord, vec2(0.5, 0.5));
  if (d < 0.5) {//https://www.2cto.com/kf/201708/647743.html
    gl_FragColor = u_FragColor;
  } else {
    discard;
  }
}